Carboxylic acids are characterized by the presence of one of more carboxyl groups in an organic molecule. The acids are usually written as RCOOH. The hydrogen atom of this group may be displaced and appear as a hydronium ion thereby justifying by theory the term acid.
Saturated linear carboxylic acids and their esters may be prepared by a large number of general procedures such as the oxidation of primary alcohols or aldehdyes, the catalytic hydrolysis of nitriles, the reaction of Grignard reagents with carbon dioxide as well as by several special procedures for specific acids including fermentation, the acetoacetic ester synthesis, the malonic ester synthesis and the Reformatsky reaction.
In recent years, with the availability of large quantities of alpha-olefins from wax cracking at relatively low costs, alpha-olefins have been considered as starting materials for fatty acid production.
The carboxylation of olefins in the presence of metal carbonyls or carbonyl precursors to produce carboxylic acids or esters is old in the literature, originally having been developed by Reppe* and his coworkers. However, the nickel and iron salts or carbonyl precursors suffer from some major drawbacks. These drawbacks include the high toxicity of the carbonyl type reagents, the production of a variety of undesirable side products due to polymerization, isomerization and reduction of the olefin substrates and most importantly, the reaction results in the production of large quantities of branched isomers in addition to the desired linear fatty acid product. FNT *This work is reviewed by C. W. Bird, Rev.62,283 (1962)
Recently, it has been shown that the production of polymeric, isomeric and reduced products can be avoided through the use of alternative homogeneous catalyst systems which are active under mild reaction conditions and which give good yields and selectivity to the desired linear fatty acids or esters. These catalyst systems consist of ligand-stabilized platinum(II) and palladium(II) halide complexes in combination with Group IVB metal halides. They are exemplified by:
PtCl.sub.2 [As(C.sub.6 H.sub.5).sub.3 ].sub.2 -SnCl.sub.2 (U.S. Pat. No. 3,819,669) and PA1 PdCl.sub.2 [P(C.sub.6 H.sub.5).sub.3 ].sub.2 -SnCl.sub.2 (U.S. Pat. No. 3,700,706). PA1 10[(C.sub.2 H.sub.5).sub.4 N[SnCl.sub.3 ]-PtCl.sub.z [ P(C.sub.6 H.sub.5).sub.3 ].sub.2 PA1 10[(C.sub.2 H.sub.5).sub.4 N][SnCl.sub.3 ]-PdCl.sub.2 [P(C.sub.6 H.sub.5).sub.3 ].sub.2 PA1 a. 10 [(C.sub.2 H.sub.5).sub.4 N][SnCl.sub.3 ]-PdCl.sub.2 [P(p-CH.sub.3.C.sub.6 H.sub.4).sub.3 ].sub.2 PA1 b. 10 [(C.sub.2 H.sub.5).sub.4 N][SnCl.sub.3]-PdCl.sub.2 [P(C.sub.6 H.sub.5).sub.3 ].sub.2 PA1 c. 5 [ClCH.sub.2 (C.sub.6 H.sub.5).sub.3 P][SnCl.sub.3 ]-PdCl.sub.2 [P(C.sub.6 H.sub.5).sub.3 ].sub.2 PA1 d. 10 [(n-C.sub.4 H.sub.9).sub.4 N](SnCl.sub.3 ]-PdCl.sub.2 [P(C.sub.6 H.sub.5).sub.3 ].sub.2 PA1 e. 5 [(C.sub.6 H.sub.5).sub.4 As](SnCl.sub.3 ]-PdCl.sub.2 [P(C.sub.6 H.sub.5).sub.3 ].sub.2 PA1 f. 5 [(C.sub.7 H.sub.15).sub.4 N][SnCl.sub.3 ]-PdCl.sub.2 [P(C.sub.6 H.sub.5).sub.3 ].sub.2 PA1 g. 10 [(C.sub.2 H.sub.5).sub.4 N][SnCl.sub.3 ]-PdCl.sub.2 [P(p-CH.sub.3 O.C.sub.6 H.sub.4).sub.3 ].sub.2 PA1 h. 10 [(C.sub.2 H.sub.5).sub.4 N][SnCl.sub.3 ]-PdCl.sub.2 [P(CH.sub.3).sub.2 C.sub.6 H.sub.5 ].sub.2 PA1 i. 5 [(C.sub.2 H.sub.5).sub.4 N][GeCl.sub.3 ]-PdCl.sub.2 [P(C.sub.6 H.sub.5).sub.3 ].sub.2 PA1 j. 5 [(C.sub.2 H.sub.5).sub.4 N][ GeCl.sub.3 ]-PdCl.sub.2 [As(C.sub.6 H.sub.5).sub.3 ].sub.2 PA1 k. 10 [(CH.sub.3).sub.4 N][SnCl.sub.3 ]-PdCl.sub.2 [P(C.sub.6 H.sub.5).sub.3 ].sub.2 PA1 l. 10 [(C.sub.2 H.sub.5).sub.4 N][SnCl.sub.3 ]-PdCl.sub.2 [As(C.sub.6 H.sub.5).sub.3 ].sub.2 PA1 a. The stabilizing ligands contain one or more phosphorous or arsenic donor atoms bonded to various alkyl, aryl or substituted aryl groups. PA1 b. The palladium halide salt is palladium chloride or bromide, and PA1 c. The quaternary ammonium, phosphonium or arsonium salts contain alkyl, cycloalkyl, aryl, substituted aryl, substituted alkyl and mixed alkaryl groups each having 1 to 12 carbon atoms, or mixtures thereof. PA1 1. a. The crude liquid ester product is separated from the product mixture by filtration, centrifugation, etc. PA1 b. The crude ester is subject to flash distillation under reduced pressure to remove unreacted olefin and excess alcohol, PA1 c. The partially purified ester is extracted with a non-polar solvent, such as petroleum ether, to precipitate dissolved melted components, PA1 d. The non-polar extract is distilled to recover residual ester, and PA1 e. The recovered melt catalyst from steps (a) and (c) is combined and recycled with additional olefin and alcohol. PA1 2. a. The crude liquid ester product is separated from the product mixture by filtration etc. PA1 b. The crude liquid product of (a) is subject to distillation under reduced pressure to strip off unreacted octene and alcohol, PA1 c. The partially purified ester is subject to fractional distillation in vacuo, PA1 d. The recovered residual melt catalyst from steps (a) and (c) is combined, and recycled with additional olefin and alcohol.
While these homogeneous catalysts are a vast improvement over the more easily poisoned and less selective heterogeneous catalysts, and the widely used highly toxic metal carbonyls such as nickel, cobalt and iron cabonyls. They also have certain drawbacks. These include difficulties in maintaining high conversions, high selectivities and high yields upon recycling the catalyst, due to catalyst degradation, and the problems of mechanical losses, and further catalysts decomposition, during the separation of the carboxylated products from the homogeneous catalysts and solvents. Catalyst instability is a particular problem with the less thermally stable palladium-containing homogeneous catalysts.